[maintenance] Add a PR template

Signed-off-by: Nick Volynkin <nick.volynkin@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Thank you for making a contribution! Here are some tips for you:
+- Start the PR title with the [label] of Cozystack component:
+  - For system components: [platform], [system], [linstor], [cilium], [kube-ovn], [dashboard], [cluster-api], etc.
+  - For managed apps: [apps], [tenant], [kubernetes], [postgres], [virtual-machine] etc.
+  - For development and maintenance: [tests], [ci], [docs], [maintenance].
+- If it's a work in progress, consider creating this PR as a draft.
+- Don't hesistate to ask for opinion and review in the community chats, even if it's still a draft.
+- Add the label `backport` if it's a bugfix that needs to be backported to a previous version.
+-->
+
+## What this PR does
+
+
+### Release note
+
+<!--  Write a release note:
+- Explain what has changed internally and for users.
+- Start with the same [label] as in the PR title
+- Follow the guidelines at https://github.com/kubernetes/community/blob/master/contributors/guide/release-notes.md.
+-->
+
+```release-note
+[]
+```